The present invention relates to an image forming apparatus, more particularly an image forming apparatus having a transfer unit on which a sheet to be image-processed is circumferentially retained.
One class of full-color copying machines has a transfer drum provided opposite to a photoconductor drum. A printing sheet is wound onto the circumferential surface of the transfer drum, wherein four successive images developed on the photoconductor drum are transferred in turn onto the printing sheet corresponding to four toner colors, i.e. yellow, magenta, cyan and black. A full-color toner image thus formed on the printing sheet is fixed in a fixing unit.
In operation, a sheet contained in a feed cassette in the full-color copying machine is transported to the transfer drum through a feed path, rotated with the transfer drum four times, and then discharged through a discharge path having the fixing unit.
If a sheet jam is detected in the feed path or the discharge path, the sheet jam is indicated on an operation panel and the copying operation is interrupted. Then, an operator removes the jammed sheet and presses on a reset switch, whereupon the copying operation is restarted.
Since high-speed type full-color copying machines handle a plurality of printing sheets concurrently, a printing sheet may exist on the transfer drum in addition to a previous sheet in the discharge path or to a subsequent sheet in the feed path. Under these circumstances, when a sheet jam is detected in the feed path or discharge path, it can happen that a further printing sheet remains on the transfer drum though the sheet jam has been cleared.
There is a sensor for detecting the presence of a sheet on the transfer drum, but in some instances the sheet as positioned on the halted transfer drum is in a region outside of that in which the sensor can detect it. As a result, restart of the copying operation is allowed even if a printing sheet is retained on the transfer drum, such that the printing sheet thus remaining becomes an obstruction to an ensuring operation.